The present invention relates to a flow injection analysis (hereinafter referred to simply as FIA) method, and more particularly to the elimination of a ghost component in such a method.
In FIA methods, a predetermined amount of a sample is injected into a continuous flow of a carrier solution so that they react with each other to result in a colored reaction product and are brought into a flow cell for spectrophotometric measurement.
In the conventional FIA method, the existing single-beam or double-beam spectrophotometer has been used so that the time-dependent variation of absorbance occurring in a flow cell is measured for one preselected wavelength. A spectrophotometric measurement system used in the conventional FIA method is shown in FIG. 1. White light energy 31 emitted from a light source 30 impinges on a light dispersion element 32 which in turn serves to separate it into various components at different wavelengths. One separated monochromatic light component 33 having a particular preselected wavelength is passed through a fixed slit 34 into a flow cell 35. The amount of light transmitted through the flow cell 35, i.e. the absorbance is detected by a photodetector 36 and the time-dependent variation of absorbance as shown in FIG. 2A is recorded on a recorder 37 such as an oscillogram. However, this conventional method involves a problem that when measurement is applied to a sample such as sea-water containing salts, ghost peaks giving an increase or decrease in apparent absorbance appear due to the refraction of light, for example, through a convex or concave lens action resulting from a difference in density between both sides of an interface (A or B in FIG. 1) of the sample and a carrier solution (such as distilled water or a reagent) so that the ghost peaks affect the measured value, thereby providing a factor of errors in measurement. FIG. 2B shows a typical example of this kind of ghost peaks which was obtained in the case where a predetermined amount of NaCl solution was injected into a continuous flow of a distilled water as the carrier solution. In such a case, it is usually assumed that no time-dependent variation of optical absorption takes place in the measurement using any wavelength in a visible and ultraviolet range since the distilled water and the NaCl solution are both colorless and clear. In actual practice, however, the ghost peaks as shown in FIG. 2B appear. In the profile shown in FIG. 2B, a negative peak is first developed and thereafter a positive peak appears. A reverse profile will be observed when the NaCl solution is used as a carrier and the water is used as a sample. If the amount of the injected sample is great, the positive and negative peaks are separated from each other. A certain combination of the carrier and the sample will give a more complicated profile. Thus, when the sample contains salts as in sea-water, the obtained time-dependent spectrum will result in a composite form of such spectrums as shown in FIGS. 2A and 2B, so that the ghost peaks constitute a factor of errors affecting the measured absorbance value.
Ghost peaks constituting a factor of errors in the FIA method also appear when a reagent as a carrier solution is colored, when fluid-flow pulsations originating from a carrier feeding pump take place, etc. Though the occurrence of the above-mentioned ghost peaks are known, any effective approach for eliminating the influence thereof has been not proposed.